1. Field of the Invention
The present invention relates to a color copying machine comprising a plurality of developing devices which contain toner of different colors, and to a method of forming a multicolored image.
2. Description of Related Art
In color copying adopting the electrophotographic processing, generally toner of three primary colors (yellow, magenta and cyan) is used, and a multicolored image including black is formed by combining toner images of these colors.
Poor combination of the toner images results in a rough, low degree of copy image.
In order to improve the quality of the copy image, U.S. Pat. No. 4,655,579 disclosed a color copying machine comprising four developing devices which contain yellow, magenta, cyan and black toner, an image sensor for reading an original image by resolving the original image into elements, and an eraser for partly erasing an electrostatic latent image on a photosensitive member. Each element of the original image is judged whether to be black, from image data collected by the image sensor. Then, in processes of forming toner images of the three primary colors, parts of the electrostatic latent image corresponding to image elements judged to be black are erased so that toner will not stick to the parts, and in a process of forming a black toner image, parts of the electrostatic latent image corresponding to image elements judged not to be black are erased.
In short, black parts of the original image are reproduced with black toner, and the other parts are reproduced by combining the three primary colors of toner.
The eraser consists of a large number of LEDs which are turned on selectively, and areas which are irradiated by two adjacent LEDs overlap so that the entire surface of the photosensitive member will have the same quantity of light from the LEDs.
When an original image composed of a color part such as a color picture, and a monochromatic part such as letters is copied by use of such a copying machine, the copy image is fine, and small letters and drawings are clear.
In this specification, "monochromatic" and "color" are not recognized merely by the eye but are recognized based on the significance of letters and patterns as information. A part composed of a field color (usually white) and a specified color (usually black) is referred as a monochromatic part, and a part including any other color is referred as a color part.
Conventionally, every black part is reproduced with black toner whether it is in a monochromatic part or in a color part, and some parts fail to be reproduced. For example, when copying a color picture wherein a black part is bordering a color part, the border area may be missing on a copy. In the processes of forming toner images of the three primary colors, a part of the electrostatic latent image corresponding to the black part is erased, and in the process of forming a black toner image, a part of the electrostatic latent image corresponding to the color part is erased. However, because parts which are irradiated by two adjacent LEDs of the eraser overlap, the overlapping portion is erased both in the process of forming toner images of the three primary colors and in the process of forming a black toner image. As a result, the overlapping portion, that is, the border area between the black part and the color part is missing on a copy.
Also, due to errors of resolution of the image sensor and errors in judging the color of an image, there have been the following problems. A color part may be reproduced in black, and blurs may occur. Black in several tones of a color part may be reproduced in uniform black. A gradation of colors around a border between a color part and a black part cannot be reproduced well.
Thus, the reproductivity of a color part is low, and a thus made copy is not identical with the original.